Environmental regulations restricting the use of octane enhancing lead additives for internal combustion engines as well as the shift in the automotive industry toward more efficient higher compression ratio engines have prompted the petroleum refining industry to seek alternate processes for meeting the demand for high octane unleaded gasoline.
In order to meet these requirements, the industry has developed non-lead octane boosters and has reformulated high octane gasoline to incorporate an increased fraction of aromatics. While these and other approaches will fully meet the technical requirements of regulations requiring elimination of gasoline lead additives and allow the industry to meet the burgeoning market demand for high octane gasoline, the impact on the cost of gasoline is significant.
Catalytic cracking processes manufacture a major segment of the total gasoline pool produced in modern oil refineries by upgrading gas oil and heavier feedstreams to a lighter product slate including gasoline and distillate as well as C.sub.4 - aliphatics rich in olefins. Examples of such catalytic cracking processes are described in P. B. Venuto and E. T. Habib, Jr., Fluid Catalytic cracking with Zeolite Catalysts (1979) as well as U.S. Pat. Nos. 2,383,686 to Wurth; 2,689,210 to Leffer; 4,093,537 to Gross et al.; 4,118,338 to Gross et al.; and 4,411,773 to Gross, Which patents are incorporated by reference herein.
To increase the overall yield of high octane gasoline from catalytic cracking units, processes have been developed which upgrade the C.sub.4 - by-products of the cracking process. With the advent of these light aliphatics upgrading processes, the demands on the catalytic cracking unit product fractionation section have also changed. Specifically, the C.sub.4 aliphatics upgrading processes operate at relatively high temperature conditions, typically above about 700.degree. F. For this reason, the H.sub.2, H.sub.2 S, and mercaptan sulfur contents of the C.sub.4 - aliphatic streams from the catalytic cracking unit product fractionation section are critical, not only to meet product specifications and to prevent accelerated catalyst deactivation, but also to assure safe and reliable unit operation using the most economical materials of construction. It has been found that levels of H.sub.2 S, H.sub.2, and mercaptan sulfur levels which were completely acceptable for lower temperature light aliphatics upgrading processes such as HF or H.sub.2 SO.sub.4 catalyzed alkylation can markedly accelerate corrosion, pitting and cracking in carbon steel and lower alloy vessels under the more severe temperature conditions associated with the catalytic upgrading processes presently under consideration. Thus it would be desirable to provide the light aliphatics upgrading process associated with the catalytic cracking unit with a C.sub.2 -C.sub.4 aliphatic stream which is relatively free from H.sub.2 S, H.sub.2, and mercaptan sulfur.
Catalytic cracking process units typically include a main fractionator, commonly called the column, which receives cooled reactor effluent from the catalytic cracking process. The main column fractionates this reactor effluent into a plurality of streams including clarified slurry oil, heavy cycle oil, light cycle oil, unstabilized gasoline and an overhead gas stream rich in C.sub.4 - olefins. The gasoline and lighter components are then further fractionated in an unsaturate gas plant which typically includes, in order, a deethanizer absorber, a debutanizer and a depropanizer.
The deethanizer absorber splits the gasoline and lighter material into a C.sub.2 - overhead gas stream and a C.sub.3 + bottoms stream. The C.sub.2 - overhead gas stream may optionally be treated in a sponge absorber to further sorb C.sub.3 + components before acidic components such as hydrogen sulfide, carbon dioxide and hydrogen cyanide are removed in a purification sorption column. Having been treated to reduce its acidic gas content, the deethanizer absorber overhead stream is then charged to a fuel gas header to be burned for fuel in the refinery complex.
The deethanizer absorber bottom stream is then charged to a debutanizer fractionator where it is split into a C.sub.5 + gasoline stream rich in olefinic components and a C.sub.3 -C.sub.4 overhead stream. The debutanizer fractionator is typically designed to meet a bottom stream gasoline volatility specification requiring vapor pressure of less than about 10 psi. Finally, the debutanizer overhead stream, rich in C.sub.3 -C.sub.4 olefins, may be fractionated into a propane/propylene overhead stream and a butane/butylene bottoms stream. This step is most often employed when additional light aliphatics upgrading capacity is available, for example, an alkylation process unit for converting iso- and normal C.sub.4 aliphatics to high octane alkylate gasoline. The C.sub.3 - depropanizer or debutanizer overhead stream may be sold as LPG, but first must be treated in a mercaptan sulfur removal process to meet sulfur content specifications. One example of such a process is the Merox process (trademark and/or service mark of UOP, Inc.).
The incremental volume of C.sub.2 - fuel gas generated by a catalytic cracking process may increase the total refinery fuel gas volume beyond that needed to fulfill its fuel gas consumption and sales requirements. To assure compliance with environmental regulations governing content and volume of gases exhausted to the atmosphere, fuel gas production is limited to the total volume which can be consumed within the refinery, sold to consumers beyond the battery limits of the refinery, or flared in accordance with the applicable environmental permits. Thus if the incremental volume of fuel gas generated by the catalytic cracking unit exceeds the capacity of facilities for its disposition, the cracking unit feedrate or reaction severity must be reduced. Neither option is economically desirable. The ideal solution would be to decrease fuel gas volume by shifting the overall yield from the catalytic cracking unit away from C.sub.2 - components and toward more valuable high octane C.sub.5 + gasoline. The acid gas components of the catalytic cracking unit reactor effluent stream tend, however, to be carried with ethane and ethylene. Clearly, then, the problem of excess fuel gas production cannot be solved merely by shifting the cut points in a conventional catalytic cracking product fractionation section because the downstream light aliphatics upgrading process would be exposed to hydrogen and acid gases under severe temperature conditions.
A number of acid gas removal processes are commercially available for treating this overhead stream including chemical solvent as well as physical sorption processes. Chemical solvent techniques include countercurrent contacting with monoethanolamine (MEA), diethanolamine (DEA) and hot potassium carbonate. Physical sorption techniques employ solid sorbents such as molecular sieves, activated charcoal and iron sponge.
Conventionally, these acid gas removal processes are installed downstream of the sponge absorber and debutanizer. Consequently, the acid gases are carried through the various upstream separation processes of the USGP including the absorber-deethanizer, sponge absorber and debutanizer. This configuration tends to increase the rate of acid gas induced corrosion of a large portion of the vessels and ancillary equipment in the USGP, leading to increased maintenance operations and plant downtime. Under the more severe temperature conditions of catalytic aliphatics upgrading processes, streams containing these acidic components readily attack carbon steel and the lower chromium- and molybdenum-containing steel alloys, and may cause cracking, pitting, blistering, or general thinning.
The available light aliphatics upgrading processes, include catalytic aromatization, oligomerization and etherification. Catalytic aromatization converts the light aliphatics over a catalyst, for example a medium-pore zeolite catalyst such as ZSM-5, to a product mixture rich in aromatics. Oligomerization and olefin interconversion may employ similar catalysts, but are typically conducted under less severe temperature conditions. Etherification reacts olefins with alcohols to form ethers useful as octane-enhancing gasoline additives. For example, isobutylene may be reacted with methanol over an acidic catalyst to produce methyl-tertiary butyl ether (MTBE) and that isoamylenes may be reacted with methanol over an acidic catalyst to produce tertiary-amyl methyl ether (TAME).
In U.S. Pat. Nos. 4,830,635; 4,826,507; and 4,788,365 to Harandi and Owen the ability of zeolite type catalyst to convert methanol to olefins is utilized by directing unreacted methanol from an etherification reaction to a zeolite catalyzed conversion reaction for conversion to olefin, thereby obviating the need to separate and recycle methanol in the etherification reaction.
The process for the conversion of methanol to olefins is but one in a series of analogous processes based upon the catalytic capabilities of zeolites. Depending on various conditions of space velocity, temperature and pressure methanol, and lower oxygenates in general, can be converted in the presence of zeolite type catalyst to olefins which may then oligomerize to provide gasoline or distillate or be converted further to produce aromatics. In another application of zeolite catalysis, light olefins can be interconverted or redistributed at low pressure and high temperature to produce higher olefins rich in isoalkenes.
Recent developments in zeolite catalyst and hydrocarbon conversion processes have created interest in using olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, and higher boiling hydrocarbon products. In addition to the basic work derived from medium pore zeolites such as ZSM-5, a number of discoveries have contributed to the development of a new industrial process, known as Mobil Olefins to Gasoline/Distillate ("MOGD"). This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.3 -C.sub.5 alkenes. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed improved processing techniques to the MOGD system as in U.S. Pat. Nos. 4,150,062; 4,211,640; and 4,227,992. The conversion of olefins to gasoline using a fluidized catalyst bed is the subject of U.S. patent application Ser. No. 006407 to Owen et al. The above identified disclosures are incorporated herein by reference. The MOGD process may produce low octane gasoline. This disadvantage requires further downstream processing of the product so produced in order to provide a gasoline product with useful road octane value. Improvement of the process to provide an instant higher octane value gasoline product has been a much sought after objective in that field of art.
These two processes, etherification and olefin oligomerization/interconversion have been advantageously integrated to provide a high octane ether-rich gasoline product from aliphatic hydrocarbon and lower alkyl alcohol feedstreams. The Mobil Olefins to Ethers and Gasoline (MOEG) process produces methyl tert-butyl ether (MTBE) and/or tertiary amyl methyl ether (TAME) by a two-step reaction sequence utilizing a catalytic etherification step as described above, followed by a zeolite catalysis to convert unreacted alcohol and olefins in the etherification effluent MTBE and TAME are formed conventionally by contacting a stream rich in isobutylene and isoamylene in the presence of a acid catalyst, e.g. a sulfonic acid resin catalyst such as Amberlyst 15, which catalyzes the iso-olefin/alcohol reaction. This process is detailed in U.S Pat. Nos. 4,830,635; 4,826,507; and 4,788,365 to Harandi and Owen, which are incorporated herein by reference for details of the MOEG process.
Thus it is clear that a process for shifting product yield in a catalytic cracking unit away from C.sub.4 - light aliphatics, particularly C.sub.2 - fuel gas, to favor production of high octane gasoline would provide substantial operational and economic benefits. Further, it would be desirable to provide the light olefin upgrading section of such a process with a feedstock of sufficient purity to meet the application environmental standards and product quality specifications while also avoiding the incremental capital costs associated with alloyed process equipment.